Clock signal distribution utilizing differential sinusoidal signal pair

ABSTRACT

A differential sinusoidal signal pair is generated on an integrated circuit (IC). The differential sinusoidal signal pair is distributed to clock receiver circuits, which may be differential amplifiers. The clock receiver circuits receive the differential sinusoidal signal pair and convert the differential sinusoidal pair to local clock signals. Power consumption and noise generation are reduced as compared to conventional clock signal distribution arrangements.

This application is a continuation of U.S. patent application Ser. No.09/682,473 filed Sep. 6, 2001, which is hereby incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

This invention is concerned with integrated circuits (ICs), and is moreparticularly concerned with distribution of clock signals on an IC.

BACKGROUND OF THE INVENTION

At the current state of the art, processors are operating at clock ratesof 1 GHz or higher, and further advances in operating rates can beanticipated. However, at current and expected future clock rates, noisegeneration and power consumption are significant issues in connectionwith clock signal distribution. For example, in one processor operatingat 1 GHz, the power consumption of the clock tree accounts for about 75%of the power consumed by the chip.

Furthermore, with conventional clock distribution techniques it has beenconsidered desirable to limit the amount of clock gating employed inprocessors operating at the high frequencies referred to above, sinceturning on and off gated clocks may result in noise spikes.

Accordingly, it would be desirable to provide clock signal distributionin a manner that accommodates very high clock frequencies with a reducednoise profile and reduced power consumption.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a method of generating a clocksignal on an IC is provided. The method includes generating adifferential sinusoidal signal pair, and generating a clock signal fromthe differential sinusoidal signal pair for the IC.

According to another aspect of the invention a method of driving a clocktree on an IC includes distributing a clock signal in the form of adifferential sinusoidal signal pair in a portion of the clock tree.

According to still another aspect of the invention, a clock circuit foran IC includes a generating circuit for generating a differentialsinusoidal signal pair, a distribution circuit coupled to the generatingcircuit for distributing the differential sinusoidal signal pair on theIC, and a plurality of clock receiver circuits coupled to thedistribution circuit for converting the differential sinusoidal signalpair into respective local clock signals.

Since a differential sinusoidal signal pair is used for distributing theclock signal, very low-swing signals may be employed, thereby greatlyeconomizing on power consumption. Moreover, since local clock signalregeneration is performed on a differential basis with reference to thedistributed pair of signals, noise which couples to both of the pair ofsignals is automatically filtered out. Also, since the clock signal issinusoidal prior to regenerating, its energy is concentrated at theclock frequency rather than being spread through the spectrum as in thecase of a square wave clock signal. Consequently, energy efficiency ispromoted.

Other objects, features and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram representation of a clock signalgeneration and distribution arrangement provided in accordance with theinvention;

FIG. 2 is a schematic diagram of a typical one of the clock receiversshown in FIG. 1; and

FIG. 3 is a waveform diagram that illustrates a differential sinusoidalsignal pair that may be generated in the clock signal generation anddistribution arrangement of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a schematic block diagram representation of a clock signalgeneration and distribution arrangement provided in accordance with theinvention.

In FIG. 1, reference numeral 10 indicates a signal generator whichgenerates a differential sinusoidal signal pair. As is well understoodby those who are skilled in the art, a differential sinusoidal signalpair comprises a pair of sinusoidal wave forms, that are substantiallyequal in frequency and amplitude but that are substantially 180° out ofphase with each other, as illustrated in FIG. 3. The differentialsinusoidal signal pair generated by signal generator 10 may, forexample, have a peak to peak differential (ppD) of about 100 mV or 150mV. The common mode level of the differential sinusoidal signal pair maybe at the center of the power supply voltage. For example, each signalof the pair may swing from about 575 mV to 625 mV when a 1.2 volt powersupply is used. It will be recognized that such a differentialsinusoidal signal pair has a peak to peak differential of 100 mV. It iscontemplated to employ a differential sinusoidal signal pair having adifferent common mode and/or a different peak to peak differential thanthe signals which have been described above.

It is well within the ability of those who are skilled in the art todesign a suitable signal generator to generate the differentialsinusoidal signal pair as described above using conventional circuitdesign techniques. Accordingly, it is not necessary to describe thedesign of the signal generator 10.

Continuing to refer to FIG. 1, distribution circuitry 12 is coupled tosignal generator 10. The distribution circuitry 12 distributes thedifferential sinusoidal signal pair from the signal generator 10 topoints on an integrated circuit at which local clock signals are to begenerated (as described below).

In order to provide for minimal skew, in at least one embodiment of theinvention it is preferred that the distribution circuitry 12 includeadjacent tracks of metal lines through which the respective signals ofthe differential sinusoidal signal pair are routed together (e.g., sothat each signal travels approximately the same distance). It is alsopreferred that there be one or two tracks of isolation between therespective tracks for the signal pair to reduce mutual couplingcapacitance. Such a routing layout can be provided using commerciallyavailable routing tools. Other routing layouts may be employed.

Because it is contemplated to use very small swing signals for thedifferential sinusoidal signal pair, the additional capacitanceresulting from two clock signal nets instead of one is more thancompensated for relative to the capacitance that would be generated by asingle full-swing digital clock.

It may be desirable for the distribution circuitry 12 to be routed andloaded so as to have inductance and capacitance that produces resonanceat the desired frequency of operation. This further reduces the clockpower requirements. Switchable loads 14 may be included in thedistribution circuitry 12 to permit the load of the distributioncircuitry 12 to be tuned to compensate for manufacturing variations.

Coupled to the distribution circuitry 12 are clock receivers 16. Thepurpose of the clock receivers 16 is to receive the differentialsinusoidal signal pair distributed through the distribution circuitry 12and to convert the differential sinusoidal signal pair to local clocksignals having a suitable wave form and amplitude. The local clocksignals may, but need not be, in the form of a square wave. The localclock signals output by the clock receivers 16 may, for example, swingfrom substantially zero volts to substantially the level of the powersupply, which may be 1.2 volts or any other suitable voltage. It willaccordingly be recognized that the peak to peak differential of thedifferential sinusoidal signal pair distributed through the distributioncircuitry 12 may be on the order of less than half or less thanone-fifth or even less than one-tenth of the amplitude of the localclock signals outputted from the clock receivers 16.

The number of clock receivers 16 may vary, and depends on thearchitecture of the IC for which clock distribution is to be provided.

The clock receivers 16 may comprise, for example, differentialamplifiers such as the differential-to-single-ended converterillustrated in FIG. 2. The circuit shown in FIG. 2 is similar toconventional differential-to-single-ended converters and accordinglyneed not be described in detail. It will be observed that the circuit ofFIG. 2 includes a first differential amplification stage 18 and a seconddifferential amplification stage 20. The first differentialamplification stage 18 and the second differential amplification stage20 are respectively dc biased by FETs 22 and 24 and are coupled withopposite polarity to the differential sinusoidal signal pair supplied atinputs 26 and 28. FET 30 is provided so that a predictable current issupplied to FETs 22 and 24 (e.g., from a current source provided viainput IREF).

Downstream from the amplification stages 18 and 20 are a thirdamplification stage 32 and output stages 34.

In the embodiment of FIG. 2, the first and second amplification stages18, 20 each comprise an NMOS differential transistor pair 18 a, 20 awith PMOS current mirror active loads 186, 206. The third amplificationstage 32 comprises a PMOS differential pair (forward from transistors 32a, 32 b) with an NMOS current mirror active load 32 c. The output stages34 comprise three conventional series-connected FET inverters 34 a, 34 band 34 c which produce complimentary outputs 35 a, 35 b. Otherdifferential transistor pair configurations and/or buffering outputstages may be employed.

In operation, the differential sinusoidal signal pair is provided to theinputs 26, 28. In response thereto, the first amplification stage 18produces a first input to the third amplification stage 32 that isproportional to the difference between the inputs 26, 28. Similarly, thesecond amplification stage 20 produces a second input to the thirdamplification stage 32 that is proportional to the difference betweenthe inputs 26, 28, but that is of opposite polarity to the first inputprovided by the first amplification stage 18. The third amplificationstage 32 comprises the difference between the first and second inputsthereto (e.g., about twice the value of the output of either the firstor second amplification stages 18, 20) to produce an output that is fedto the output stages 34. The output stages 34 further amplify the outputof the third amplification stage 32 to produce complimentary outputs 35a, 35 b that fully transition between ground (e.g. V_(SS)) and the powersupply rail (e.g., V_(DD)).

Because the inputs 26, 28 to the clock receiver circuit 16 shown in FIG.2 are routed together, any noise experienced by one of the inputs isalso experienced by the other input, and such noise is filtered by theclock receiver circuit 16 because it is a differential amplifier.Because of the noise immunity provided by the clock receiver circuits16, the signals distributed to the distribution circuit 12 may have verylow voltage swings, which results in reduced power dissipation.

If the clock receiver circuit 16 shown in FIG. 2 is balanced, then theoutput signals have 50% duty cycle. Ideally, the outputs 35 a, 35 b arenon-overlapping (e.g., the output 35 a is never high at the same timethat the output 35 b is high, or vice versa, as the outputs 35 a, 35 bare complimentary. It is also contemplated to create imbalances in theclock receiver circuit 16 of FIG. 2 by, e.g., arranging the dimensionsof pairs of FETs to be different within one or more of the amplificationstages 18, 20, 32 to create a non-50% duty cycle. In general, anytechnique that can controllably introduce asymmetry to differentialamplifier transistor pairs may be similarly employed (e.g., manipulatingdoping levels, threshold voltage implant doses, etc.). In this way, thepurely differential input could be converted to a set of non-overlappingclock signals, which would be suitable for use with latches that callfor non-overlapping clocks.

Those who are skilled in the art will recognize that many modificationsand variations of the circuit shown in FIG. 2 would be suitable for useas the clock receivers 16 of FIG. 1. Design of this or other suitabletypes of clock receiver circuitry is well within the ability of thosewho are skilled in the art.

Referring once more to FIG. 1, clock splitters 36 are provideddownstream from clock receivers 16. In one embodiment of the invention,about ten clock splitters 36 are coupled to each clock receiver 16.Other numbers of clock splitters may be coupled to each clock receiver16. The clock splitters 36 are provided for further distribution of theregenerated clock signals output from the clock receivers 16 (as isknown in the art). The clock splitters 36 also generate non-overlappingclocks for latches which call for such signals. Any conventional clocksplitters may be employed for the clock splitters 36.

Because of the low noise profile of the clock distribution systemillustrated in FIG. 1, it is feasible to employ a substantial amount ofclock gating. The clock gating may be implemented, for example, byselectively gating off the clock receivers 16 using any conventionalclock gating technique.

From the foregoing description it will be understood that all of thesignal generator 10, the distribution circuitry 12, the clock receivers16 and the clock splitters 36 may be formed on an integrated circuit(IC) 38 such as a microprocessor. It will also be recognized that thecircuitry between signal generator 10 and the outputs of clock splitters36 constitutes some or all of a clock tree.

With the clock signal distribution arrangement provided in accordancewith the present invention, power consumption is reduced and noisegeneration is minimized, as compared to conventional clock signaldistribution techniques. The present invention also accommodates clockgating and generation of a set of non-overlapping clocks.

The foregoing description discloses only the exemplary embodiments ofthe invention; modifications of the above disclosed apparatus andmethods which fall within the scope of the invention will be readilyapparent to those of ordinary skill in the art. For example, althoughthe present invention is particularly suited for application tomicroprocessors, it may also be used in connection with any other typeof integrated circuit for which clock distribution is employed.

While the present invention has been disclosed in connection withexemplary embodiments thereof, it should be understood that otherembodiments may fall within the spirit and scope of the invention, asdefined by the following claims.

1. A method of driving a clock tree on an integrated circuit (IC), themethod comprising the steps of: providing an IC having a clock tree;distributing a clock signal in the form of a differential sinusoidalsignal pair in a portion of the clock tree, the differential sinusoidalsignal pair comprising a first sinusoidal signal and a second sinusoidalsignal; and generating a local clock signal from the differential pairby employing both the first sinusoidal signal and the second sinusoidalsignal to form the local clock signal; wherein generating the localclock signal includes using a differential amplifier.
 2. The method ofclaim 1, wherein the differential pair has a peak to peak differentialof not more than about 150 mV.
 3. The method of claim 2, wherein thedifferential pair has a peak to peak differential of substantially 100mV.
 4. The method of claim 1 wherein the local clock signal has anamplitude substantially equal to a power supply voltage, and thedifferential sinusoidal signal pair has a peak to peak differential thatis substantially less than the amplitude of the local clock signal. 5.The method of claim 1, wherein the local clock signal is a square wavesignal.
 6. A clock circuit for an IC, comprising: a generating circuitadapted to generate a differential sinusoidal signal pair comprising afirst sinusoidal signal and a second sinusoidal signal; a distributioncircuit coupled to the generating circuit and adapted to distribute thedifferential sinusoidal signal pair on the IC; and a plurality of clockreceiver circuits coupled to the distribution circuit and adapted toconvert the differential sinusoidal signal pair into respective localclock signals by employing a respective differential amplifier and boththe first sinusoidal signal and the second sinusoidal signal to formeach local clock signal.
 7. The clock circuit of claim 6, wherein thelocal clock signals have an amplitude substantially equal to a powersupply voltage, and the differential sinusoidal signal pair has a peakto peak differential that is substantially less than the amplitude ofthe local clock signals.
 8. The clock circuit of claim 7, wherein thepeak to peak differential of the differential sinusoidal signal pair isless than half the amplitude of the local clock signals.
 9. The clockcircuit of claim 8, wherein the peak to peak differential of thedifferential sinusoidal signal pair is less than one-fifth the amplitudeof the local clock signals.
 10. The clock circuit of claim 9, whereinthe peak to peak differential of the differential sinusoidal signal pairis less than one-tenth the amplitude of the local clock signals.
 11. Theclock circuit of claim 6, wherein the differential pair has a peak topeak differential of not more than about 150 mV.
 12. The clock circuitof claim 11, wherein the differential pair has a peak to peakdifferential of substantially 100 mV.
 13. The clock circuit of claim 6wherein the distribution circuit includes means for tuning a frequencyresponse of the distribution circuit.
 14. The clock circuit of claim 6,wherein each local clock signal is a square wave signal.